Low-density porous expanded polytetrafluoroethylene (PTFE), described in U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, 4,187,390, 4,902,423, and 4,478,665, has been widely used to insulate electrical conductors to provide insulated conductors having improved properties of velocity of signal propagation, dielectric loss, and physical dimensions as compared to conductors insulated with full density polymer insulation. The high pore volume and low-density provide the improvements in the properties.
A limitation to achieving extremely high signal propagation velocity through such insulated conductors lies in the open cell (nodes and fibrils) nature of ePTFE which is not inherently crush-resistant when it is manufactured to have a very high void content or pore volume to achieve low-density and low dielectric constant and therefore high velocity of signal propagation.
Crushability of such an insulation can be improved by enclosing the insulation with a skin of thermoplastic polymer, but the velocity of signal propagation is reduced by the solid voidless insulation skin.
Another method for providing crush-resistance to the cable insulation has been to foam thermoplastic polymers as they are being extruded around a conductor to yield a crush-resistant closed cell foam insulation around the conductor. The method is well known in the art and described in U.S. Pat. Nos. 3,072,583, 4,711,811, and 4,394,460 and in EP0442346 in which a foaming gas or liquid is injected into the molten polymer during extrusion. In these methods a foaming agent is used during the extrusion process to yield closed cell fluorocarbon polymer foams, which tend to be inherently crush-resistant. It is difficult, however, to produce a foam insulation having a high enough void content to provide insulated cables having high signal velocity propagation through them and at the same time provide adequate resistance to crushing.